Positively guided valve drive

ABSTRACT

In a positively guided valve drive of an internal combustion engine including a cam having a flexible belt wrapped around it and being pivotally connected to a valve shaft, the flexible belt is connected to the valve shaft by means of a carriage which has three parallel shafts two of the shafts being connected to the ends of the flexible belt and the third shaft being connected to the valve shaft for operating the valve.

This is a Continuation-In-Part Application of international application PCT/EP03/04895 filed May 10, 2003 and claiming the priority of German application 102 25 620.9 filed Jun. 7, 2002.

BACKGROUND OF THE INVENTION

The invention relates to a positively guided valve drive, in particular for an internal combustion engine of a motor vehicle, including a camshaft with cams and flexible straps which extend around the cams and are connected to the valve shafts by means of connecting elements engaging the straps.

Positively guided valve drives, referred to as desmodromic valve drives, are characterized in that a cam element is rotatably arranged in a flexible wraparound element which is in turn connected to the valve which is movable in a plane perpendicular to the axis of rotation of the cam element. The wrap around element engages the valve for opening and closing the valve.

WO 01/12958 A1 discloses a positively guided valve drive in particular for an internal combustion engine of motor vehicles, which has a driven cam element, a valve actuating element which can be displaced or pivoted by the cam element and a flexible wrap-around element. The cam element is rotatably arranged in the wrap-around element. The wrap-around element is also connected to the valve actuating element by means of a joint so as to be capable of moving in a plane perpendicular to the axis of rotation of the cam element.

Basically the following technical problems have arisen with such a valve drive. The distance between the continuous wrap-around element and the surface of the cam should be the same for any cam angle. If this distance varies, for example, owing to the type and method of connection, different wrap-around lengths occur depending on the position of the wrap-around element relative to the cam. The degree to which the wrap-around element length varies depends directly on the contour of the cam, the type of valve connection and the resulting distance between the wrap-around element and the surface of the cam.

A further problem is posed by the connection of the valve to the wrap-around element. Depending on the desired transmission function of the cam and the associated valve strokes and accelerations, there is a more or less distinct to and from movement of the joint for every revolution of the cam, as a result of which there is a one-sided rolling movement around the joint. This rolling movement means, of course, that the wrap-around element is shortened. This in turn causes the wrap-around element to become blocked on the cam as soon as the shortening is greater than the play present between the cam and wrap-around element permits.

In addition, as a result of the rolling movement a large amount of flexing work is applied to the wrap-around element, which causes to high wear.

The object of the invention is therefore to provide an improved, positively guided valve drive with which the wrap-around element can be reliably prevented from self-locking and at the same time a partial compensation of the lengths of the wrap-around element can be ensured as a function its respective position in relation to the cam element.

SUMMARY OF THE INVENTION

In a positively guided valve drive of an internal combustion engine including a cam having a flexible belt wrapped around it and being pivotally connected to a valve shaft, the flexible belt is connected to the valve shaft by means of a carriage which has three parallel shafts two of the shafts being connected to the ends of the flexible belt and the third shaft being connected to the valve shaft for operating the valve.

Using a connecting element between the wrap-around element and the valve actuating element on which, apart from the shaft for the valve actuating element, two further shafts for fastening the connecting element to the wrap-around element are provided, makes it possible to use open wrap-around elements. Furthermore, during operation, the wrap-around element can partially unwind from, or wind onto, the shafts. As a result, the valve drive is prevented form self-locking and operation with wrap-around elements with a high consistency of length becomes possible. The wrapping onto, or unwrapping from, the shafts also leads to a shortening of the changes between the wrap-around lengths. This in turn is a precondition for the formation of a uniform lubricating gap between the wrap-around element and the cam element, thus ensuring hydrodynamic lubrication.

The construction of the connecting element from two end areas of the wrap-around element and three shafts which are arranged between the end areas provides for a very simple design.

Adapting the radius of curvature of the connecting element to the smallest possible radius of curvature of the cam element ensures that the connecting element at this point fits as snugly as possible to the cam element, thus, further reducing the changes of the wrap-around length.

By providing slots in the shafts of the connecting elements, through which the open ends of the wrap-around element extend and where they are secured, a simple and efficient way of mounting is implemented. Providing a rounded contour in the entry regions of the slots reduces the wear on the wrap-around element.

By using a polymer tie belt with vulcanized-in tie lines it is possible for the variable wrap-around lengths already to be compensated on the basis of the resiliency of the wrap-around element.

The invention will become more readily apparent from the following description of a preferred embodiment thereof described below, by way of example only, with reference to the accompanying drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a perspective view of a first exemplary embodiment of a positively guided valve drive including a cam

FIG. 1 b shows the valve drive with the cam removed,

FIG. 1 c shows the wrap-around strap attached to a connecting element,

FIG. 1 d shows the wrap-around strap and the connecting element with the front plate of the connecting element removed,

FIG. 1 e shows the wrap-around element alone,

FIG. 2 is a side view of a connecting element alone,

FIG. 3 is a perspective view of an exemplary embodiment of one of the shafts of the connecting element shown in FIG. 1,

FIG. 4 shows a cross-section through the shaft,

FIG. 5 illustrate the cam movement over the connecting element,

FIG. 6 shows a particular structure of a wrap-around element, and

FIGS. 7 a and 7 b show without the cam and with a cam of a valve drive with a wrap-around element according to FIG. 6.

DESCRIPTION OF PARTICULAR EMBODIMENTS

In FIG. 1, the positive valve drive is indicated overall by the numeral 1. The arrangement of such a valve drive 1 in internal combustion engines is generally known to the person skilled in the art and is therefore not described in more detail. The valve drive 2 has a valve stem actuating structure 2 with a valve plate 3 and a valve stem 4. Furthermore, the valve drive 1 includes a cam element 5 which is provided for operating the valve actuating element 2 and is connected in the customary way to a camshaft of the internal combustion engine so as to be fixed for rotation therewith. For this purpose, the cam element 5 has, in its end face 6, a bore 7 through which the camshaft extends in the installed state. The longitudinal axis of the camshaft at the same time also defines the axis 8 of rotation of the cam element 5. In order to save weight, the cam element 5 can additionally have one or more cutouts 9.

The external contour 10 of the cam element 5 is surrounded by an open, flexible wrap-around element 11. The wrap-around element 11 is closed off by a connecting element 12 which at the same time forms the connection to the valve actuating element 2. The design of such a connecting element 12 is shown in a plan view in FIG. 2. The connecting element 12 is composed of three shafts 13-15 and two side parts 16, 17. The external contour 18 of the two side parts 16 adjacent the cam element 5, is preferably adapted to the contour of the cam in such a way that the smallest radius of curvature of the cam element 5 fits snugly to the curvature of the side parts 16, 17. The shaft 15 serves as an axis of rotation between the wrap-around element 11 and valve actuating element 2. This shaft 15 is preferably arranged in the region between the two shafts 13, 14 and has a larger distance from the external contour 18 with respect to the shafts 13, 14. The two shafts 13, 14 are each permanently connected to one end of the open wrap-around element 11. In the process, both frictionally locking and positively locking types of connections between the ends of the wrap-around element 11 and the axles 13, 14 are possible. For example, in the form of welding, soldering, wedging, screwing or wrapping around. In this way, the connecting element 12 functions as the connecting clip of the wrap-around element 11. A precondition for a functional system is the possibility of both ends of the wrap-around element 11 of wrapping around, or unwrapping from, the respective shafts 13, 14 of the connecting element 12 by a few angular degrees.

The valve actuating element 2 is pivotally connected to the shaft 15. The shaft 15 is arranged parallel to the axis 8 of rotation of the cam and so arranged that the axis of the valve stem 4 always intersects the axis of the camshaft irrespective of the position of the cam element 5. For the sake of connection, an intermediate element 19 is provided which is permanently connected to the valve stem 4 and is at the same time mounted so as to be rotatable about the shaft 15. However, in principle, it is also possible for a bearing for the shaft 15 to be arranged directly on the valve stem 4.

In particular in internal combustion engines with a relatively long valve stroke it may be necessary to provide an additional lateral force support. For this purpose, in the exemplary embodiment according to FIG. 1 in each case a guide 20 is arranged on the intermediate part 19 on opposite sides in the direction of the axis 8 of rotation of the cam. These guides are guided parallel to the valve stem 4 in rails which are part of the cylinder head.

The valve drive 1 is illustrated in FIG. 1 b without the cam element 5 so that the guide of the wrap-around element 11 with respect to the connecting element 12 can be seen more satisfactorily. The wrap-around element 11 is guided with the respectively open ends between the external contour 10 of the cam element 5 and the respective shaft 13, 14, the wrap-around element 11 being wrapped around the shafts 13, 14 in opposite directions at least over part of the circumference. The wrap-around element 11 is connected, fixed in terms of rotation, to the two shafts 13, 14, the shafts 13, 14 also being in turn connected, fixed in terms of rotation, to the side parts 16, 17. As a result, when there is a relative change in position between the wrap-around element 11 and the connecting element 12, the wrap-around element 11 is wound onto, or unwound from, the respective shaft 13, 14. A preferred attachment method of the wrap-around element 11 to the shafts 13, 14 and to the side parts 16, 17 is described below with reference to FIGS. 3 and 4. From FIG. 1 b it is also apparent that the intermediate part 19 is wrapped around the shaft 15 at its end facing away from the cam element 5.

The design of the connecting element 12 is apparent from FIGS. 1 c and 1 d, the valve actuating element 2 including the intermediate part 19 not being illustrated in FIG. 1 c, whilst in FIG. 1 d, the side part 17 is also removed. FIG. 1 e finally shows only the wrap-around element 11.

Spring steels of a high strength may be used for the wrap-around element 11. However, owing to the high carbon content, they are suitable only to a very limited degree for a thermally joined connection between the wrap-around element 11 and connecting element 12. One advantageous solution is a winding technique in which the wrap-around element 11 is wound onto the respective shaft 13, 14.

As is illustrated in FIGS. 3 and 4, the shafts 13, 14 may be provided for this purpose with a central slot 21 through which the open ends of the wrap-around element 11 extend. On the opposite side, the wrap-around element 11 is then preferably permanently connected to the respective shaft 13, 14 by a thermal joining method, for example, soldering, laser welding or electron beam welding, and as a result is prevented from slipping out. With this type of connection, it is particularly necessary to ensure that the wrap-around element 11 is oriented at a precise right angle to the respective shaft 13, 14. furthermore, the shafts 13, 14 have a highly rounded contour 22 on their sides opposite the respective joint 23 at the entry region of the slot 21 so that the wrap-around element 11 is prevented from bending.

During the manufacture of the valve drive 11, the wrap-around element 11 is firstly brought to the necessary length and connected in a frictionally looking fashion to the two shafts 13 and 14.

The uniform winding of the wrap-around element 11 onto each of the two shafts 13, 14, is then carried out in a mounting tool (not illustrated). For this purpose, the wrap-around element 11 with the two shafts 13, 14 attached to it and the two side parts 16, 17 including the axle 15 are inserted into the tool. At the same time, the wrap-around element 11 is placed over the cam element 15 or preferably over an equivalent base element. During the circumferential configuration of the base body, it is necessary to pay particular attention to the wrap-around length which is different depending on the position of the cam element 5, and to select the largest occurring wrap-around length plus the desired winding length as the circumference. The winding itself is then carried out until the wrap-around element 11 rests uniformly on the base body. For the unwinding process, the shafts 13, 14 have circular bearing sections 24 which are rotatably guided in corresponding bores in the side parts 16, 17. Furthermore, the shaft 13, 14 have a profile 25 at least at one axial end for the unwinding process, which process 25 can be used to rotate the shafts 13, 14 relative to the side parts 16, 17. If the wrap-around element 11 has been wound on in the desired fashion, a preferably thermal joining process (for example, laser welding) by means of which the shafts 13, 14 are connected to the side parts 16, 17 in a frictionally locking fashion is carried out. As a result, unwinding or turning back is prevented.

The wrap-around element 11, which is mounted to the connecting element 12, is pushed directly from the mounting tool onto the cam element 5, or onto a mandrel with the same circumference for the purpose of support. The number of windings on each of the shafts 13, 14 is dependent in particular on the tensile force which is applied later to the wrap-around element 11, the coefficient of friction on the surface of the wrap-around element 11 and its width. In order to set the coefficient of friction on the surface of the wrap-around element 11 as high as possible, a coating with corresponding layers in the region of the wound-on length can be provided. For example, particles of diamonds in a chemical nickel matrix are used for this purpose. Furthermore, the maximum transmissible force is highly restricted by the thickness of the transmission element 11. However, this is also limited by the otherwise excessively high rigidity. Nevertheless, if a relatively high force is required this may be brought about by a multilayer structure.

The valve movement is explained in more detail below with reference to FIG. 5. Here, the position of the connecting element 12 relative to the axis 8 of rotation of the cam is illustrated schematically in different angular positions of the cam element 5. In order to simplify the illustration, the valve actuating element 2 is not shown. The possible direction of movement of the valve actuating element is defined by the imaginary connecting line between the axis 8 of rotation of the cam and the shaft 15. This means that the valve actuating element and thus also the shaft 15 can move along this imaginary connecting line 26 only in the vertical direction. At the same time, the movement of the cam element 5 is determined by the position of the axis of rotation of the cam. Since the wrap-around element 11 is in turn permanently connected to the connecting element 12 and rests on the external contour of the cam element 5, the cam element 5 thus rotates relative to the wrap-around element 11 or connecting element 12. If the process is considered from the point of view of a viewer who is rotating with the cam, the connecting element 12 is pulsed by the wrap-around element 11 like a carriage over the external contour 10 of the cam element 5.

With the rotation of the cam element 5, the force to open the valve is transmitted by the cam element 5 into the valve stem 4 via the side parts 16, 17 and shaft 15. The closing process takes place by virtue of the fact that the cam element 5 pulls on the two axles shafts via the wrap-around element 11 and as a result initiates the closing movement of the valve stem 4 via the shaft 15. The braking of the valve stem 4 just before it comes to rest on the valve seat (not illustrated) then in turn takes place directly by means of the cam element via the two side parts 13, 14 and the shaft 15.

The maximum valve stroke h is defined by the different positions of the shaft 15, that is, a position where the connecting element 12 rests against the basic circle of the cam element 5 (far left on the drawing). At the same time, it is apparent that the connecting element 11 fits snugly to differing degrees against the external contour 10 of the cam element 5 as a result of the curvature of the side parts 16, 17 and depending on the position of the cam element 5. In a flat region of the cam edge (second figure from left), the connecting element 12 fits least snugly against the outer contour 10, while in the region with the greatest curvature (second figure from right) the connecting element 12 bears directly against the cam element 5. At the same time, the angle of inclination between the connecting element 12 and the valve stem 4 or between the external contour 10 of the cam element 5 and the connecting element 12 in the region of the shafts 13, 14 also changes. As a result of this relative rotation between the connecting element 12 and external contour 10 of the cam element 5, the wrap-around element is rolled onto the respective shaft 13, 14 or unrolled from it. As a a result, despite the change to the wrap-around length and a relative rotation, self-locking of the wrap-around element 11 is prevented. This means that it is actually this rolling on movement or unrolling movement which permits such valve drives 1 to operate with wrap-around elements with a high degree of consistency of length.

At the same time, this device has the advantage that the connecting element serves at the same time as a closure for the open wrap-around element so that such wrap-around elements can be manufactured from the finite piece of belt material. Furthermore, the reduction in the variance of the wrap-around length by means of the cam element provide the precondition for a uniform lubrication gap to be formed between the wrap around and cam element, thus permitting improved hydrodynamic lubrication.

A second exemplary embodiment of a valve drive according to the invention is shown by FIGS. 6 and 7, identical parts being designed by identical reference symbols. In contrast to the first exemplary embodiment, the wrap-around element (11) includes bearing sections 27, 28 at the open ends, which sections are attached to the axles 13, 14 when the valve drive is mounted. The wrap-around element 11 is preferably composed of a polymer tensile belt, preferably neoprene or polyurethane, with a vulcanized-in tie line 29. Tie lines may be, for example, steel braided conductors or glass fibers or aramide or Kevlar fibers which are mounted in an infinite screw shape.

The bearing sections 27, 28 are used to apply force to the wrap-around element 11. The bearings sections 27, 28 are embodied, for example, as steel sleeves around which the tie lines 29 are wound and subsequently vulcanized-in. furthermore, regions 30, which include filler material or damping material can also be provided between the bearing sections 27, 28 and the tie lines 29. In principle, a closed or an open design is possible, the open, finite design having significant advantages in terms of manufacture and mounting capability in conjunction with the connecting element 12. During the final mounting process, the belt is placed around the cam element 5 and closed by the connecting element 12. As a result, the bearing sections 27, 28 are pushed onto a respective shaft 13, 14 and then secured against slipping out by attaching the second side part 17. The closure is carried out in the cam position with the minimum wrap-around element 5 must also be present in this position. However, the open wrap-around element 11 can also be connected to the connecting element 12 using other suitable means. The arrangement of the attachment points (shafts 13, 14) in relation to the axle 15 on the connecting element 12 is essential.

In general terms, the use of a polymer tie belt provides the advantage of a high degree of flexibility and adjustable extension within predefined limits. Owing to the extension capability of the polymer tie belt, it is already possible to partially compensate the variable wrap-around length. 

1. A positively guided valve drive (1) for an internal combustion engine comprising at least one driven cam element (5), a valve actuating element (2) actuated by the cam element (5), a flexible wrap-around element (11) extending around the cam element (5)), a connecting element (12) connected to said actuating element (2) and having a connecting shaft (15) which extends parallel to an axis (8) of rotation of the element (5), the valve actuating element (2) being pivotally connected to the connecting element (12) by the connecting shaft (15), and the connecting element (12) having two additional shafts (13, 14) which extend parallel to the connecting shaft (15), the wrap-around element (11) being of open design and having opposite ends, each attached to one of the additional shafts (13, 14).
 2. The valve drive as claimed in claim 1, wherein the connecting element (12) comprises two spaced side parts (16, 17) which are essentially flat, are arranged parallel to one another and are connected to one another by the connecting shaft (15) and the two additional shafts (13, 14) which are all arranged perpendicularly to the side parts (16, 17).
 3. The valve drive as claimed in claim 1, wherein the side parts (16, 17) of the connecting element (12) are curved at their ends facing the cam element (5) with a curve radius corresponding essentially to the smallest curve radius of the cam element (5).
 4. The valve drive as claimed in claim 1, wherein the ends of the open wrap-around element (11) can be wound onto, and unwound from, the respective additional shafts (13, 14).
 5. The valve drive as claimed in claim 1, wherein the connecting shaft (15) and the additional shafts (13, 14) are arranged on the connecting element (12) in such a way that the additional shafts (13, 14) have a shorter radial distance from the axis (8) of rotation of the cam than the connecting shaft (15).
 6. The valve drive as claimed in claim 1, wherein the wrap-around element (11) is a finite metallic tie strap.
 7. The valve drive as claimed in claim 6, wherein the additional shafts (13, 14) each have a slot (21) through which the wrap-around element (11) extends and is connected to the additional shafts (13, 14) in a frictionally locking fashion.
 8. The valve drive as claimed in claim 7, wherein the additional shafts (13, 14) have entry regions at the slots (21) with a rounded contour.
 9. The valve drive as claimed in claim 1, wherein the wrap-around element (11) is embodied as a polymer tie belt with vulcanized-in tie lines (29).
 10. The valve drive as claimed in claim 9, wherein the wrap-around element (11) has bearing sections (27, 28) at the open ends for attachment to the additional shafts (13, 14) of the connecting element (12).
 11. The valve drive as claimed in claim 10, wherein regions (30) which are provided with filling material or damping material are provided between the bearing sections (27, 28) and the tie lines (29). 